For gravity, it's only true for spherical shells, but in the case of charged electrical conductors, it's true no matter what the shape of the shell is. The field inside a charged, conductive small stellated dodecahedron is also zero everywhere inside. That's because the charge in the conductor migrates as long as there's an electrical field to make it migrate; it stops moving once it neutralizes the field.

(If you made an actual small stellated dodecahedron with infinitely sharp points, the charge would leak away from the points via field emission. Or, in theory, you'd have an infinitely strong electrical field there, and therefore all kinds of singularities. But reasonable approximations of the setup are possible in real life.)